Development Of Multi-Phase Variable Frequency Drives For The Oil & Gas Industry

Abstract

Liquefied Natural Gas (LNG) plants rely heavily on LNG compressors exceeding 100 MW power range. These compressors are mainly driven by gas turbines and high power helper motors operated via Variable Frequency Drives (VFDs). The VFDs offer a wide range of control and flexibility to the typically large motors allowing the driven system to meet desired process operation requirements in the LNG plants. Such motor drive systems have the capability to meet the power demand required for starting very large LNG compressors and provide additional power (helper) when necessary. High performance VFDs contribute to a great extent in enhancing the performance of LNG compressors and their reliability. Various types of VFDs are available in the oil and gas industry. Their performance and reliability depend on the topology of the VFD system. To satisfy the requirement of oil and gas applications and obtain the highest dynamic performance of the drive, special considerations are taken in choosing the appropriate drive topology. This thesis presents and discusses the design of a high power multilevel and multi-phase motor drive system suitable for LNG compressor applications. Specifically, a 5-phase 5-level Neutral Point Clamped (NPC) based high power motor drive system is proposed. The proposed system was designed for a 33 MW 6.3 kV induction motor and its performances were tested and simulated under different operating conditions. It showed superior results compared to the widely and commonly used 3-phase 3-level NCP inverter. The produced phase voltage has five levels, which had reduced the voltage and current Total Harmonics Distortion (THD) significantly and provided smoother output torque with minimized torque ripples